1. Field of the Invention
The present invention relates to a microfilm search device which determines the presence of frames from a density change in the microfilm running direction, particularly to a microfilm search device which detects a frame dimension or a dimension between frames from a density change in the microfilm running direction to determine the presence of frames and search for a desired frame. The present invention also relates to a method of detecting a frame dimension or a dimension between frames from a density change in the microfilm running direction.
2. Description of the Related Art
In a known microfilm search method, document or image marks (blips) are photographed or recorded besides the frames on microfilm and used as reference marks. The blips of microfilm during running are read and counted, and the blip count is used to identify frame addresses when a specific frame is sought.
On the other hand, there is proposed a search method in which, instead of using the blips, the presence of frames is directly detected, and a desired frame is searched for from a sequence of detected frames. Specifically, a density sensor is disposed within the travel width of the frame, so that the presence of frames is determined from a change in film density detected by the density sensor.
In order to detect the presence of frames in this manner, at least one pair of optical fibers are provided whose end faces are opposed to each other with a microfilm placed therebetween. The light is emitted from one optical fiber and transmitted through a film. The light transmitted through the film is received by the other optical fiber, and the quantity of received light is detected by a photosensor. In this case, a signal (density signal) indicating the film density detected by the photosensor is binarized with a predetermined threshold value into a white or black signal. The white signal represents a film transparent portion and the black signal represents a film non-transparent portion. Based on the white or black signal(s), a frame or an area between frames (between-frames) is detected. For example, in a negative film, since the background of a frame is recorded as black and the between-frames is recorded as white (transparent), the binarized density signal indicates black for the frame and white (transparent) for the between-frames. The binarized signal turns to logic ZERO ("0"; level L) inside each frame (black), and turns to logic ONE ("1"; level H) between the frames (white).
It is then determined that the area where the binarized signal becomes logic ZERO is the frame while the area where the binarized signal becomes logic ONE is the between-frames. Specifically, while the logic ZERO or ONE is maintained, sampling (pulse) signals outputted from an encoder are counted to detect a length in which the logic ZERO or ONE is obtained on the film. Subsequently, by comparing the detected length (frame dimension, or dimension between frames) with the predetermined set value (set range), the presence of frames can be determined.
However, the diameter of the optical fiber is finite length and cannot be infinitely reduced. In additions, the light is diffused during the light is emitted from the optical fiber on the light emitting side, passes through the film and reaches to the optical fiber on the light receiving side. As a result, the frame dimension or the dimension between frames cannot be accurately determined by using a change of the density signal obtained by the photosensor. Therefore, there is a problem that it is difficult to determine the presence of frame with high accuracy and, accordingly, the accuracy of searching for a desired frame is lowered.
Moreover, in the detection of frames, if the frame contains therein a null area, i.e., a portion having substantially the same density as that of an area between frames (between-frames), this portion is erroneously determined as the between-frames. For example, when the microfilm is a negative film, a background portion in the frame on which image or document is photographed becomes black (non-transparent), while a peripheral portion surrounding the frame is white (transparent). When black characters or figures are depicted on a white background of the original, the character or graphic region becomes transparent (null) in the frame. Accordingly, when the presence of frames is determined from a density change on the microfilm, the transparent (null) area other than a between-frames is incorrectly determined as the between-frames. This causes an error in counting the number of frames, and therefore, correct frame search cannot be performed.
To solve the problem, there may be proposed a method in which the length of the transparent null area is detected. Specifically, when the detected length is within the set range, the transparent area is determined as a null area. When the detected length is beyond the set range, the transparent area is determined as a between-frames. In this case, however, if the film feeding direction is reversed during the detection of the transparent portion in the frame, the length of the transparent portion may be detected longer than actual. For example, if the film feeding direction is reversed while the length of the white (transparent) area is counted, the length is counted twice at maximum as long as the actual length. Therefore, the transparent null area may be incorrectly determined to be the between-frames. Since the number of frames is counted more than actual, the desired frame cannot be correctly sought.